DEPR: DatetimeIndex.snap()

[jreback]

we have a DatetimeIndex.snap, this is duplicative of our more general .ceil(), .floor() and .round() (and .to_period().to_timestamp()); the former are also generally available on all of the timelike indices (DTI, TDI, PI)

we should remove this and prefer the 3 rounding methods

Corollary: isn't normalize just floor('D')?

To be clear, round/floor/ceil do not currently accept non-ticks while snap() does. I've started on a fix for those methods, but no responses.

Until that's fixed, deprecating would mean deleting useful functionality.